A dielectric may be coated using various deposition processes. In an electroless process, the dielectric may be coated using a controlled autocatalytic (self-continuing) reduction, e.g., by subjecting the dielectric to a reducing chemical bath of various materials with metal, such as copper, nickel, silver, gold, palladium, or others. Adhesion of an electroless-deposited material to a dielectric surface may be primarily mechanical. Consequently, electroless deposition may be most effective when the dielectric surface is relatively rough, e.g., with critically needed placed anchor points. However, to reduce electrical losses, it is desirable to reduce the dielectric-metal interface roughness.
In a physical vapor deposition (“PVD”) process, a desired film material such as titanium may be vaporized and deposited onto a dielectric surface inside of a vacuum. In some PVD processes, the vapor may be applied to the dielectric surface as condensation. In other PVD processes known as “sputtering,” the desired film material (sometimes referred to as the “target material”) may be bombarded by energetic particles. This causes atoms to be ejected from the target material and projected onto the dielectric surface to form a thin coating. In contrast to electroless deposition, PVD deposition, particularly sputtering, works best on a relatively smooth dielectric surface.
Dielectric surface smoothness is governed largely by the curing process, which is performed after a thin film such as a laminate is removed from the dielectric surface. Heating ramp rate, maximum temperature and cure time may all be tailored to achieve a desired smoothness. However, the level of smoothness that may be achieved using these curing variables is limited. Moreover, downstream processes such as chemical desmearing (e.g., to evacuate a drilled opening or “via”) may increase average surface roughness (Ra) of exposed dielectric surfaces, e.g., from 140 nanometers (“nm”) to greater than 350 nm.